Citation

Abstract / Synopsis

Producing high quality soybean seed in the hot humid tropics is no easy task. During
seed production, several environmental factors and plant morphological characteristics
can exert their influences on seed quality. A study was undertaken at Universiti Putra
Malaysia to study the effect of canopy architecture and seasonal variations on several
seed quality attributes in soybean (GZycine max L. Merr.). Four soybean cultivars
namely, Palmetto, AGS190, Deing and Cikurai were grown in the field for four seasons
during 2003 and 2004. Four levels of defoliation treatments [0% defoliation
(*170.89pmol m'2s" light intensity), 25% defoliation (*324.33pmol m'2s-' light
intensity), 50% defoliation (*473.01pmol m-2s-' light intensity) and 75% defoliation
(+642.84pmol m'2s" light intensity], were iniposed at the pod initiation stage (R3).
Weather factors such as light intensity, canopy, air and soil temperatures, canopy and air
relative humidity, soil moisture and leaf area index were recorded at seven-day intervals
starting from the imposition of defoliation until plants reached physiological maturity
(R7). Seeds harvested at harvest maturity (R8) were used to determine seed yield,
viability, vigour, 1 00-seed weight and for Phomopsis bioassay. Phomopsis sp. seed
infection was predicted using weather factors and leaf area index. Scanning electron
microscopy (SEM) was used to study the progression and colonization of Phomopsis sp.
on the stem, pod and seed starting from R3 until R8.
Defoliation treatments were found to have inconsistent effects on seed yield (kg ha-')
and pods per plant for AGS190, Deing and Palmetto. Pod number of Palmetto was
affected during season Ill only whereas the pod number for Cikurai was significantly
affected for all seasons. However, defoliation treatments affected 100-seed weight for all
cultivars except for AGS 190 (season I), Deing (season IV) and Cikurai (season 111).
From the combined analysis of data over the four seasons, defoliation improved percent
seed germination from 6.8 to 13.2%. Increasing the level of defoliation resulted in
increasing percent germination and 3-day seedling height and reduced Phomopsis
incidence for all cultivars. The highest germination was recorded during season I1 which
coincided with the least level of Phomopsis sp. seed infection. Moreover, this disease
was influenced by seasonal variations. The disease incidence was high during seasons Ill
(51.3%) and IV (49.5%) characterized by high rainfall during seed development and
maturation as compared to seasons I and 11 which encountered low rainfall situations
(33.3% and 32.5%, respectively). AGSl90, a large seeded cultivar, was severely
affected by the seed-borne disease (51.5% infection) whereas Deing, a small seeded
cultivar, was the least affected (34.7%).
Defoliation treatments increased light intensity within the plant canopy for all cultivars
studied. Light intensity and canopy temperature revealed negative correlation with
percentage Phomopsis incidence indicating that high light intensity and temperature
inside plant canopy reduced Phomopsis sp. seed infection. On the contrary, positive
relationship was observed between canopy and air relative humidity, soil moisture and
leaf area index with percentage Phomopsis incidence. From the stepwise multiple
regression analysis, Phomopsis sp. seed infection can be predicted by leaf area index,
soil moisture and canopy relative humidity; while its reduction can be predicted by
increased light intensity and canopy temperature during seed development and
maturation.
Scanning electron microscopy revealed that hngi progression and colonization started at
different growth stages for different plant parts. Stems were infected during the early
reproductive stage (R3) whereas pods became noticeably infected during the full seed
stage (R6) and seeds were the last to be infected. The hngi were internally- borne within
the infected seeds. Fungal hyphae were observed externally on the surface of the plant
parts studied and internally both in the pod and in all the three layers of the seed coat:
palisade cell, hourglass cell and parenchyma cell layers. Fungi colonization was highest
at the late plant growth stages of R7 and R8.
A progressive increase in pod and seed infection was detected during subsequent growth
stages between R6 and R8. From the pod, the pathogen can infect and colonize the seed.
The SEM results suggested that the reproductive growth period of R6 to R7 was more
critical with respect to Phomopsis sp. seed infection than earlier reproductive growth
periods, since more severe colonization of pods and seeds took place at the later stages
of plant growth. SEM revealed that stem infection allowed buildup of inoculum for
subsequent infection of the pod, whereas pod infection was necessary for further
infection of seeds. Since pod infection is the prerequisite for seed infection, this study
suggests that fbngicide would be best applied between R4 and R5 before the seed-borne
fungi reach the seed during R6 and the subsequent growth stages.
Prediction model based on four seasons' data accurately described the relationship
between the environmental conditions and leaf area index during seed development and
maturation and the levels of seed infection by Phomopsis sp. Moreover, the model fitted
well with the field and laboratory data collected. However, this model needs to be tested
at multilocational trials for validity.
The results of the present study have shown that plant canopy modification through
defoliation appears to improve quality of seeds produced under wet and warm tropical
environments. In addition, the study also suggested that growing of soybean cultivars
with open canopies and having low leaf area index, coupled with rain-free harvesting
seasons can result in the production of high quality seeds. Although the prediction model
so developed in this study needs to be tested for validity at different locations and
variable environments, it has the potential to be used as a practical tool in plant disease
forecasting programs.